In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacing and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or anti-arrhythmia therapies to the heart at a desired energy and rate. A cardiac stimulation device is electrically coupled to the heart by one or more leads possessing one or more electrodes in contact with the heart muscle tissue (myocardium). One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
A stimulation pulse delivered to the myocardium must be of sufficient energy to depolarize the tissue, thereby causing a contraction, a condition commonly known as “capture”. In early pacemakers, a fixed, high-energy pacing pulse was delivered to ensure capture. While this approach is straightforward, it quickly depletes battery energy and can result in patient discomfort due to extraneous extracardiac stimulation, e.g., of surrounding skeletal muscle tissue, the patient's phrenic nerve or the patient's diaphragm.
The “capture threshold” is defined as the lowest stimulation pulse energy at which capture occurs. By stimulating the heart chambers at or just above this threshold, comfortable and effective cardiac stimulation can be provided without unnecessary depletion of battery energy. The capture threshold, however, is extremely variable from patient-to-patient due to variations in electrode systems used, electrode positioning, physiological and anatomical variations of the heart itself, and so on. Furthermore, a capture threshold may vary over time within a patient as, for example, fibrotic encapsulation of an electrode can occur after implantation of the electrode.
Implantable lead(s), attached to an implantable pulse generator (IPG), such as a pacemaker and/or implantable cardioverter defibrillator (ICD), is/are used to deliver such stimulation pulses to the myocardium. Some such leads are Multi-Electrode Leads (MELs), meaning they include multiple electrodes for use in pacing and/or sensing. MELs allow for more flexibility in pacing and sensing, as compared to single electrode leads. Generally, the more electrodes on a lead, the more flexibility provided. For example, one lead design includes four electrode arrays (also referred to as groups or bands) with four electrodes each, thus resulting in a single lead with sixteen electrodes. An example of a lead that can include sixteen (and even more) electrodes is disclosed in U.S. Patent Publication No. 2006/0058588 (U.S. patent application Ser. No. 11/219,305), entitled “Methods and Apparatus for Tissue Activation and Monitoring” (Zdeblick), published Mar. 16, 2006 (filed Sep. 1, 2005), which is incorporated herein by reference.
With some MELs, such as the MEL described in Zdeblick, one or more commands can be sent to control circuitry of the MEL to configure the electrodes. A configuration of the MEL can have one or more of the electrodes connected as an anode, one or more of the electrodes connected as a cathode, and other of the electrodes disconnected. After the MEL is configured, the connected electrodes can be used for pacing and/or sensing. The same multi-conductor bus in the MEL can be used for configuring the electrodes, for pacing and for sensing.